The present invention relates to a multi-color electroluminescent device including multi-component conductive polymer layers capable of voltage-dependent, tunable multicolor electroluminescent emissions.
Conjugated polymers in their native state are molecular semiconductors which are of growing interest in optoelectronic and electronic devices, including light-emitting diodes (LEDs) (Friend et al., 1999; Kraft et al., 1998; Tarkka et al., 1996; Eichen et al., 1998; Burn et al., 1993; Zhang et al., 1998(a)), lasers (Tessler et al., 1996; Hide et al., 1996; Wegmann et al., 1998), photovoltaic cells (Antoniadis et al., 1994(a); Halls et al., 1995), xerographic imaging photoreceptors (Zhang et al., 1996; Zhang et al., 1997(a); Osaheni et al., 1994(a); Osaheni et al., 1994(b); Abkowitz et al., 1992; Antoniadis et al., 1993), and thin film transistors (Garnier, 1998). In the case of polymer LEDs, synthetic manipulation of macromolecular architecture has made available diverse light-emitting conjugated polymers from which LEDs of various colors have been fabricated and are now being optimized by a variety of device engineering strategies. In efforts to design next generation electroluminescent (EL) materials with significantly improved EL efficiency and to explore novel phenomena, i.e., multicolor emission (Berggren et al., 1994; Jenekhe et al., 1997; Zhang et al., 1998(b); Hamaguchi et al., 1996; Wang et al., 1997), exciplex emission (Jenekhe et al., 1994; Osaheni et al., 1994(c); Gebler et al., 1997; Gebler et al., 1998), and photon harvesting/energy transfer (Yang et al., 1994; Lee et al., 1996; Tasch et al., 1997), not feasible in conjugated homopolymers, multicomponent conjugated polymer systems, which include multilayered thin films (Jenekhe et al., 1997; Zhang et al., 1998(b); Hamaguchi et al., 1996; Wang et al., 1997; Fou et al., 1996; Onitsuka et al., 1996; Strukelj et al., 1995; Li et al., 1997; Greenham et al., 1993; O""Brien et al., 1996; Yamamoto et al., 1996; Cui et al., 1999; Dailey et al., 1998), blends (Berggren et al., 1994; Yang et al., 1994; Lee et al., 1996; Tasch et al., 1997; Chen et al., 1997(a); Jenekhe et al., 1996; Yu et al., 1995; Zhang et al., 1997(b)), and block copolymers (Chen et al., 1996; Wagaman et al., 1997; Chen et al., 1997(b); Chen et al., 1997(c)), are of increasing interest.
In the simplest polymer LED, an EL polymer thin film, such as poly(p-phenylene vinylene) (PPV), is sandwiched between two electrodes of different work functions as schematically shown in FIG. 1a. Such a single-layer polymer LED is generally inefficient for two principal reasons. First, there is poor charge injection at one or both metal/polymer interfaces due to the inability to simultaneously match the anode work function ("PHgr"a) to the highest occupied molecular orbital (HOMO) and the cathode work function ("PHgr"c) to the lowest unoccupied molecular orbital (LUMO) of the polymer. The energy barriers to hole and electron injection at the anode and cathode are respectively xcex94Eh (="PHgr"axe2x88x92IP) and xcex94Ee (="PHgr"cxe2x88x92EA) where IP is the ionization potential and EA is the electron affinity of the polymer (FIG. 1). Secondly, there is a huge disparity between hole and electron mobilities in semiconducting polymers (Antoniadis et al., 1994(b); Blom et al., 1996; Lin et al., 1996(a); Lin et al., 1996(b)), thus precluding balanced charge transport in the devices. Commonly studied EL polymers such as PPV (Friend et al., 1999), polyphenylenes (Leising et al., 1996; Leising et al., 1997), polyfluorenes (Grice et al., 1998; Pei et al., 1996; Lee et al., 1999), polythiophenes (Berggren et al., 1994) and their derivatives are p-type (hole transport) polymers which have hole mobilities that are orders of magnitude larger than electron mobilities, relatively small bafflers to hole injection from indium-tin-oxide (ITO, "PHgr"axcx9c4.7-4.8 eV) (Kugler et al., 1997), and very large bafflers to electron injection from air stable cathodes such as aluminum ("PHgr"cxcx9c4.0-4.3 eV) (Weast et al., 1987-1988).
Two-layer polymer/polymer heterojunction LEDs have been found dramatically to improve EL efficiency and brightness (Jenekhe et al., 1997; Zhang et al., 1998(b); Strukelj et al., 1995; Li et al., 1997; Greenham et al., 1993; O""Brien et al., 1996; Yamamoto et al., 1996; Cui et al., 1999; Dailey et al., 1998), compared to the one-layer devices (FIG. 1). This is consistent with findings in multilayered organic/organic diodes (Tang et al., 1987).
n-Type (electron transport) polymers used in such two-layer heterojunction LEDs are thought to improve device efficiency through their high electron affinities which reduce the barrier to electron injection at the cathode/polymer interface (Greenham et al., 1993). An increasing part of current EL materials research effort is thus being directed to the design and synthesis of n-type polymers with improved properties (Strukelj et al., 1995; Li et al., 1997; Greenham et al., 1993; O""Brien et al., 1996; Yamamoto et al., 1996; Cui et al., 1999; Dailey et al., 1998). Both non-conjugated polymers, such as the oxadiazole-containing side-chain polymers (Strukelj et al., 1995; Li et al., 1997), and xcfx80-conjugated polymers such as polycyanoterephthalylidenes (CN-PPVs) (Greenham et al., 1993), polyphenylquinoxalines (O""Brien et al., 1996; Yamamoto et al., 1996; Cui et al., 1999), polypyridines (Dailey et al., 1998), and polyquinolines (Jenekhe et al., 1997) have been reported as electron transport layers in two-layer heterojunction LEDs. What is currently lacking, however, is understanding of the roles of the electronic structures and sizes of the polymer/polymer interfaces in such two-layer heterojunction LEDs. In contrast, extensive studies of metal/polymer interfaces (Salaneck et al., 1996; Kugler et al., 1999; Gao, 1999) in LEDs have provided knowledge of their general features and properties in relation to device performance. For example, the indium-tin-oxide (ITO)/PPV interface is believed to be quasi-ohmic, if not ohmic, for hole injection (Antoniadis et al., 1994(c)), whereas the cathode (Al, Ca, Mg)/PPV interface injects electrons by tunneling and/or other complex processes (Parker et al., 1994). Al/PPV interface is known to exhibit Schottky barrier characteristics, leading to photovoltaic properties (Antoniadis et al., 1994(a)).
In addition to their possible important roles in the two-layer heterojunction LEDs, polymer/polymer interfaces can also play a critical role in even single-layer LEDs if the polymer layer consists of a phase separated blend (Berggren et al., 1994) or a microphase separated block copolymer. More generally, polymer/polymer interfaces mediate a variety of photophysical and charge transfer processes in multicomponent conjugated polymer systems exemplified by efficient energy transfer in binary nanophase separated blends (Yang et al., 1994; Lee et al., 1996; Tasch et al., 1997) and block copolymers (Chen et al., 1996), exciplex formation (Jenekhe et al., 1994; Osaheni et al., 1994(c); Gebler et al., 1997; Gebler et al., 1998) in bilayers and blends, ground-state electron transfer in binary blends (Chen et al., 1997(a)), photoinduced electron transfer in binary blends (Jenekhe et al., 1996), and tunable multicolor electroluminescence in bilayers (Jenekhe et al., 1997; Zhang et al., 1998(b); Hamaguchi et al., 1996; Wang et al., 1997) and blends (Berggren et al., 1994). The coupling of finite size effects to the electronic structure and properties of polymer/polymer interfaces in such multicomponent polymers has been suggested from observed multicolor EL emission from two-layer heterojunctions (Jenekhe et al., 1997). Conjugated polymer bilayer heterojunctions have also been extensively studied as rectifying junctions (charge trapping electrodes, charge storage) in electrochemical experiments (Torres et al., 1990; Hillman et al., 1990).
The prior art has failed to define criteria for structural assembly and selection of compatible polymers for multi-color electroluminescent devices. The present invention overcomes these deficiencies in the art.
The present invention relates to an electroluminescent device including an anode and a cathode capable of being electrically connected to a power supply and a voltage regulator; and a multi-layered polymer structure, between the anode and cathode, including a first polymer layer which includes a hole transfer polymer contacting the anode and a second polymer layer which includes an n-type conjugated polymer contacting the cathode, wherein changes in the voltage of current passing through the electroluminescent device change the color of electroluminescent emissions from the multi-layered polymer structure.
Another aspect of the present invention relates to a full color display including a plurality of pixels, each pixel including an electroluminescent device of the present invention.
Yet another aspect of the present invention relates to a method of making a multi-color electroluminescent device, the method including the step of forming, between first and second electrodes, a multi-layered polymer structure including a first polymer layer which includes a hole transfer polymer and a second polymer layer which includes an n-type conjugated polymer.
The present invention identifies the role played by the electronic structure of a polymer/polymer interface and its affect on the EL diode efficiency and luminance. Finite size effects on the polymer/polymer bilayer heterojunctions were also explored. The substantially planar heterojunction is an ideal model system for investigating the electronic structure and properties of polymer/polymer interfaces; well-defined planar heterojunctions of diverse conjugated polymers and layer thicknesses can be prepared by a number of polymer processing techniques. The layer thicknesses of p-type polymer and the n-type polymer in a bilayer heterojunction were varied to probe size effects and multicolor EL emission. In addition to spectroscopic and electrical measurements we also used electroluminescence microscopy to characterize the heterojunction LEDs. The systematic investigation of the electroluminescence of bilayer heterojunctions of hole transfer polymers (p-type layer) and a series of n-type conjugated polymers, with electron affinities in the 2.36-4.0 eV range and ionization potentials in the 5.06-5.90 eV range, has shown that the electronic structure of the polymer/polymer interface plays a more important role on EL efficiency and diode brightness than injection barrier at the cathode/polymer interface. For a p-type emissive layer, such as PPV, the present invention demonstrates that both efficiency and diode brightness are maximized when the energetics of the bilayer heterojunction interface favors electron transfer while disfavoring hole transfer. A similar requirement of favorable hole transfer and unfavorable electron transfer across the interface of a bilayer LED with an n-type emitter also follows. Therefore, although synthesis of n-type (electron transport) polymers with large electron affinities is of broad interest per se, for applications in LEDs the absolute LUMO/EA and HOMO/IP energy levels of the n-type polymer are not the most critical, but these energy levels relative to those of the p-type (hole transport) polymer in a heterojunction are.
Continuous voltage-tunable multicolor emission was observed in bilayer heterojunction LEDs containing emissive p-type and n-type layers with sizes in the range of 20-50 nm. However, bilayer heterojunction LEDs of similar composition but having thicker layers had conventional single-color emission. These results show that the electronic and optical properties of polymer/polymer heterojunctions critically depend on the relative sizes of the bilayer components. These finite size effects originate from the small charge carrier ranges in semiconducting polymers. These observations on the roles of polymer/polymer interfaces and on the associated finite size effects on the electroluminescence of bilayer heterojunctions are expected to be applicable to other multicomponent conjugated polymer systems.